Discover how to measure battery state-of-charge and what future developments may bring.
One of the main tasks of the smart battery is to establish communication between the battery and the user. A fuel gauge indicating state-of-charge fulfills part of this effort. When pressing the TEST button on a fully charged SMBus battery, all signal lights illuminate. On a partially discharged battery, half the lights illuminate, and on an empty battery all lights remain dark or a red light appears. Figure 1 shows a fuel gauge of a battery that is 75 percent charged, with three lights glowing.
Figure 1: State-of-charge readout of a “smart” battery
Signal lights indicate the battery SoC when pressing the TEST button.
Courtesy of Cadex
While the SoC information displayed on a battery or a display screen is helpful to the user, the readout does not guarantee the expected runtime. The fuel gauge resets to 100 percent on a full recharge regardless of how much capacity the battery can store. A serious miscount occurs if an aged battery shows 100 percent SoC while the battery’s ability to hold charge has dropped to 50 percent or less. We ask, “100 percent of what?” If, for example, 100 percent of a good battery results in a four-hour runtime, a battery holding half the capacity would run for only two hours. The user should know that the fuel gauge only shows SoC; capacity, the leading health indicator, remains unknown.
Other than doing a full discharge with a controlled current and measuring time, there is no reliable method to calculate the state-of-health (SoH) of a battery. A full discharge is normally done as part of a planned maintenance and calibration procedure by removing the battery from the device. However, there is a digital way to estimate the capacity of a smart battery on the fly, and many systems offer this feature.
At time of manufacture, the SMBus battery is programmed with a specified capacity, which is 100 percent by default, and the battery keeps this information as permanent data. With each full charge, the battery resets to the charge flag; and during discharge the coulomb counter measures the consumed energy. A perfect battery would deliver 100 percent on a calibrated fuel gauge. As the battery ages and the capacity drops, the delivered energy between charges decreases. The discrepancy between the factory-set 100 percent and the delivered coulombs after a full charge can thus determine the battery capacity.
Coulomb counting can also estimate SoH during charging, and this works best with a fully discharged battery. A battery with a 100 percent capacity will receive the full coulomb-count; one with only 50 percent will accept only half the coulomb count before the battery reaches full-charge because there is less to fill. Inaccuracies occur by not knowing the exact state-of-charge (SoC) at the beginning of the coulomb count and the open circuit voltage (OCV) measurement only gives a rough estimation. Temperature and deployed cathode material in Li-ion also affects the OCV.
The SoC and capacity information can be shown on a linear display using colored LEDs. The green lights indicate the usable capacity; the empty part of the battery is marked with dark LEDs; and the unusable part is shown with red LEDs. Figure 2 illustrates a tri-state fuel gauge. Alternatively, the results can be a shown on a digital display.
Figure 2: Tri-state fuel gauge.The tri-state fuel gauge reads the “learned” battery information on the SMBus and displays it on a multi colored LED bar. The illustration shows a partially discharged battery of 50% SoC with 20% empty and 30% unusable.
Courtesy of Cadex
Installed in an SMBus charger or the battery pack, the tri-state fuel gauge provides state-of-function (SoF), the ultimate battery diagnostics. SoF gives the battery user a clear indication of SoC and when to retire a pack based on the encroaching unusable part that is growing with usage and age.
Device manufacturers are hesitant to install SoF into a consumer device that is readily accessible by the user. This is understandable because a battery ages, even during the warranty period, and the user would press for a new battery before the warranty expires. Device manufacturer are obliged to furnish a replacement battery if the capacity drops below 80 percent. Keeping this information hidden is seen as least disruptive. If made available, SoF would in most cases only be accessible by service personnel.
Cars with electric propulsion systems use advanced fuel-gauge technologies to determine the state-of-charge and state-of-function of the battery. The challenge is in knowing how far a vehicle can travel with a fully charged battery in various ambient conditions. A tailwind on a sunny day is more forgiving than a headwind with snow. The aging of a battery adds to the challenge and the fuel gauge loses accuracy over time. Showing 30 minutes of remaining runtime and then shutting down, as is common with laptops, will not be easily forgiven with the electric vehicle.
Last updated 4/3/2015
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